1. Field of the Invention
This invention relates to improvements of the electromagnetic drive of the focal plane shutter.
2. Description of the Prior Art
As the strength of magnetization in permanent magnets has recently increased, an increasing number of shutters employing electrically operated magnet devices as the drive source have come to be known. Of such electromagnetically driven shutters there is one which comprises a coil or other electrically conducting member in a magnetic field of a stator when current is supplied which rotates in either direction depending on the direction of current flow and which is drivingly connected through its shaft to shutter blades. In this shutter, however, it has been very difficult to establish a desired running speed curve of the shutter blades. In application of such an electromagnetic drive device to slit shutters, which provide for the leading and trailing curtains with respective individual electromagnetic drive sources, another problem which arises is that there is a possibility of an uneven exposure across the picture frame when the leading and trailing curtains unintentionally have different running curves from each other due to size variations from part to part or differences in the intertia and sliding friction between the leading and trailing curtains.
In the conventional or spring powered type slit shutter, such a problem has been solved by adjusting the diameter of the wire, the number of turns, and the convolution diameter of the spring for each of the leading and trailing curtains followed by finely adjusting the drive spring characteristic when the number of revolutions of the drive shaft in charging is changed, so that the aforesaid variations and the differences are absorbed and corrected.
However when the electromagnetic force is used as the drive force, to adjust the weight distribution, friction and other factors of each of the curtains to specific settings is technically a very difficult operation, and moreover, is also not amenable to mass production techniques. For this reason, in the art of conductive rotor type electromagnetically driven shutters, a method for previously adjusting the phase of rotation of the rotor so that the flux density of the magnetic field formed between the rotor and the stator of the permanent magnet varies as a prescribed function of the phase has been proposed in U.S. Pat. No. 4,417,797.
The present invention relates to one aspect of the above-identified method and more particularly to improvements of the method for adjusting the angular position of the permanent magnet. This adjusting method has a problem in that when turning the permanent magnet, it must be grasped at the outer periphery by a tool. Because the outer periphery is of an arcuate shape, the clamping force in the diametrical direction is necessarily far greater than the tangential force necessary to turn the permanent magnet stator against frictional force on the mount. Meanwhile, to increase the rate of the driving power to the current intensity of the battery, it is usual to use rare earth magnet as the substance of the stator. When this permanent magnet stator while being grasped by pliers is being turned to perform the adjusting operation, a crack or a broken piece is often produced, causing imperfections in that part. Also, because the broken piece is attracted to and held on the permanent magnet itself, and since the yoke, stator and rotor are positioned as close to each other as possible to improve the rate of the driving power, the electrically conducting member of the rotor is damaged when the rotor comes into contact with the broken piece as it rotates.
To avoid this, a pair of holes may be formed in the magnet to engage an adjusting tool. For this purpose, if the machining method is employed, special means such as a grinder is necessary. Since the magnet is made of brittle material such as rare earth, the production cost is increased. In the worst case, no machining means can be employed.
Another problem arising in the assembly line of such an electromagnetic drive device is that if the stator after magnetization is brought into assembly, the attraction force working on the yoke and other metal parts will make it very troublesome to perform the assembling operation. To avoid this, there has been a proposal for magnetizing the stator after it has been assembled. By this proposal, however, when assembling, the stator must be accurately oriented with respect to the direction of magnetization. Otherwise, the efficiency of later magnetization would become worse, causing the characteristics of the resultant permanent magnet to be badly deteriorated. And, this direction of magnetization is very difficult to detect because it is not yet magnetized itself. To reduce this difficulty, for example, it must be weakly magnetized before assembling. Then after determination of the direction has been made, de-magnetization is carried out before it is assembled. But, accuracy of direction determination is not very good.
The present invention is to improve conventional drawbacks as described above and to mold a pair of adjusting holes into a permanent magnet in the round flat surface portion thereof in symmetrical relation to the center of the area when the permanent magnet is formed by sintering techniques.
Because this round flat surface does not utilize the flux of the permanent magnet, a sufficiently long gap to the moving coil may be provided so that even when a broken piece remains attached on the permanent magnet, there is little possibility of damaging the moving coil. Another advantage is that since the two adjusting holes are symmetrically positioned to each other with respect to the center of rotation of the permanent magnet, no grasping force is required at the center of rotation of the permanent magnet and no grasping force is required to be applied to the permanent magnet when the adjusting operation is performed. Thus, the percentage of permanent magnets which have been assembled without any cracked broken away portions and other imperfections is reduced.
And, since the adjusting holes can be formed by using a common mold for the permanent magnet stator when sintering, almost no increase in cost will result. Furthermore, the use of the sintering technique in forming the permanent magnet allows for application of a magnetic field to the particles of raw material in the mold to orient the polarity of each particle to a direction in which the permanent magnet is to be magnetized before the sintering is carried out. Therefore, the sintered mold has a shape so that a line passing through the two adjusting holes makes a precise angle with the direction of magnetization. Therefore, if this angle is exactly zero or 90.degree., it is easy to determine in what direction the permanent magnet be oriented when it is to be magnetized.